Various inorganic materials are now widely employed for industrial use in consideration of both property of the material and requirement of the use. A silicic ceramic material, for example silicon carbide, or silicon nitride, is excellent in mechanical strength, chemical resistance, and thermal stability. A silicic material such as silicon oxide, and titanium oxide additionally has excellent optical properties.
Since the inorganic material is hard and brittle, it is generally difficult to mold and to process the inorganic material. The inorganic material is also poor in adhesiveness with an organic material, and therefore the use is generally restricted.
On the other hand, the organic polymer is flexible and is easily processed. However, their hardness and thermal stability are largely inferior to those of the inorganic material.
Therefore, there is great demand for the material which is able to complement to each other in their properties, and is able to make the use of the respective advantages thereof.
For solving the problem, a reinforcing filler such as glass fibers, glass beads, silica, alumina, and calcium carbonate is usually included or dispersed in an organic polymer material. Such an organic-inorganic composite material has been investigated in order to add excellent properties of an inorganic material such as hardness, strength, heat resistance, and weather resistance to an organic polymer material. However, an inorganic material is generally immiscible with an organic polymer material, and it is not easy to control a dispersion state of the inorganic material microscopically.
When an inorganic material is included in an organic polymer material, a large quantity of the inorganic material have to be dispersed finely and homogeneously in an organic polymer material in order to modify the organic material effectively. Whereas, if the particle size of an inorganic material becomes small, the inorganic material becomes easy to agglomerate in an organic polymer material. Therefore it is difficult to disperse fine particles of inorganic material into an organic polymer material at random with aggregation.
Furthermore, there is a maximum limit of an addition amount of the inorganic material. Thus, if the addition amount is increased beyond the maximum limit, molding property of the resulting composite material becomes poor, fracture or cracks may easily occur in the resulting composite material.
As described above, the method of blending or combining an inorganic material with an organic polymer, is not sufficient, and it is desired to provide a novel means for providing a high-performance organic-inorganic composite material.
As a means for solving the problem, organic-inorganic hybrid polymer materials are studied. The organic-inorganic hybrid polymer material is the polymer material in which an inorganic element such as Si, Ti, and Zr is incorporated in a backbone frame of an organic material. The material is generally prepared by using sol-gel reaction with a metal alkoxide compound. The inorganic element is covalently bonded to the backbone frame of the organic material, and a dispersed state of the inorganic element becomes molecularscopically homogeneous throughout the material.
Japanese Patent Kokai Publication No. 43679/1993, 86188/1993, 104710/1996, 104711/1996, Macromolecules, vol. 25, page 4309, 1992, J. Inorg. Organomet. Polym., vol. 5, page 4, 1995, J. Appl. Polym. Sci. vol. 58, page 1263, 1995, and the like disclose an organic-inorganic hybrid polymer material in which a vinyl polymer or a hydrophilic polymer is employed as an organic polymer material.
On the other hand, functionally graded materials have recently been investigated in the art. The functionally graded materials is the high-performance material in which a compositional ratio or component distribution is gradiently altered throughout the material. The functionally graded materials is expected to be applied in the art of aircraft, aerospace, nuclear fusion, electronics, medical, and the like. Although functionally graded materials have heretofore been mainly investigated by using metal materials or ceramics, functionally graded materials by using organic polymers are also recently reported.
For example, Japanese Patent Kokai Publication No. 138780/1993 describes a plastic molded product of which heat resistance is gradiently distributed, prepared by radically curing plural layers, each of the layers being composed of radically polymerizable vinyl polymers having different viscosities.
Japanese Patent Kokai Publication No. 57009/1994 describes a polyolefin of which crosslinking degree is gradient, prepared by mixing, fusing and molding with changing the ratio of an alkenyl silane/olefine copolymer component to a catalyst component. Japanese Patent Kokai Publication No. 176325/1997 describes a material in which a silicon or oxygen content is gradient, prepared by heat treating a polymer having a Si—H bond and alkyne.
Japanese Patent Kokai Publication No. 283425/1996 describes an example for applying the technique of the functionally graded materials to an organic-inorganic composite material. There is described in the publication, a polymer material with compositional gradient in which metal oxide particles are dispersed in an organic polymer. The component-gradient polymer material is prepared by the process comprising: applying a homogeneous solution of a heat curable resin composition and silicone alkoxide onto a substrate; hydrolyzing and polycondensing the silicone alkoxide under a specific condition; and curing the heat curable resin. However, the metal oxide particles and the organic polymer are not covalently bonded in the polymer material, and the polymer material with compositional gradient disclosed herein is classified into a dispersed type organic-inorganic composite material.
Further, a metal oxide content of the polymer material with compositional gradient is up to about 60%. Such a level of the metal oxide content is insufficient as functionally graded materials. On the other hand, such an amount is thought to be about a maximum limit for dispersing an inorganic material into an organic polymer material. Therefore, it is difficult to further increase an amount of metal oxide particles dispersed in the component-gradient polymer material.
The technical effects of the functionally graded materials are generally heat shock resistance, warpage resistance, and the like. However, Japanese Patent Kokai Publication No. 283425/1996 does not refer to such a properties.